CSS Rules and Regulations

So, I'm building a tool to help me format/clean up my old messy css and hopefully force myself to be better as CSS.

In testing my output I've been using CSS Lint Online (https://csslint.net/) and have found that there are rules in how your CSS should be formatted.

Thus far I've broken the "only declare h1-6 tags once" and the "put properties in alphabetical order". My tool fixes these for me now.

To save me some time in discovering more rules, what are the CSS rules that you live by and must never be broken?